PIR sensor need help

Discussion in 'The Projects Forum' started by ericyeoh, Jan 21, 2011.

1. ericyeoh Thread Starter Active Member

Aug 2, 2009
58
0
Guys i still dun understand the band pass design in below attrached circuit.

the 1st IC1A and IC1B opamp hv , FL and FH at 1.59hz.

FH = (1/[2(pie) 10mic x 10K]) = 1.59hz
FL = (1/[2(pie) 0.1mic x 1M]) = 1.59hz

how come both cut off frequency at 1.59hz? if this is the case, the bandwidth =0 hz?

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2. wayneh Expert

Sep 9, 2010
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I'm no expert on filters, but they're not like a brick wall. Wouldn't that arrangement give a peak transmittance at 1.59 Hz and taper off on both sides?

3. ericyeoh Thread Starter Active Member

Aug 2, 2009
58
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But as i studied about PIR sensor, the radiance produce by human carry around 0.1-10hz with 1mV-20mV waveform.

4. frpr666 Active Member

Feb 2, 2010
50
8
Use free simulator LT Spice. It is really great.

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5. wayneh Expert

Sep 9, 2010
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Perfectly consistent then. Filter out everything that's not near 1Hz ± factor of 10

6. ericyeoh Thread Starter Active Member

Aug 2, 2009
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wayneh, i cann;t catch what u meant. Could you please explain further?

7. GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
The IR radiation is not what this circuit is about. These sensors are typically dual (opposing) elements. The circuit senses a rate change across the elements. The filtering aspects of the circuit are designed around this rate of change. Motion from a radiant object has a very low frequency compared to circuit noise, which the latter must be filtered out.

8. wayneh Expert

Sep 9, 2010
13,624
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Combining a low pass and a high pass filter gives you a band-pass filter. Frequencies too high or too low are attenuated. An audio amplifier is sort of a band-pass filter, since it allows frequencies in the 20-20kHz range to pass, but not much outside that range. Of course that's a 1000X range, quite wide and a challenge to achieve.

In your case the band allowed to pass is centered at 1.59Hz, and the width of the band allows a range of nearby frequencies to also pass. But frequencies too high or low - things not related to a human - are filtered out.